<template>
  <ul class="list">
    <li
      class="item"
      v-for="(item) of letters"
      :key="item"
      @click="handleClick"
      @touchstart.prevent="handleTouchStart"
      @touchmove="handleTouchMove"
      @touchend="handleTouchEnd"
      :ref="item">
      {{item}}
    </li>
  </ul>
</template>

<script>
  export default {
    name: 'Alphabet',
    props: {
      cities: Object
    },
    computed: {
      letters () {
        const letters = []
        for (let i in this.cities) {
          letters.push(i)
        }
        return letters
      }
    },
    data () {
      return {
        touchStatus: false,
        startY: 0,
        timer: null,
      }
    },
    //加载数据的时候 执行
    updated () {
      this.startY = this.$refs['A'][0].offsetTop//元素距离顶部的高度
    },
    methods: {
      handleClick: function (e) {
        //向父亲传值
        this.$emit('change', e.target.innerText)
      },
      handleTouchStart: function () {
        this.touchStatus = true
      },
      handleTouchMove: function (e) {
        if (this.touchStatus) {
          if (this.timer) {
            clearTimeout(this.timer)
          }
          //优化执行频率
          this.timer = setTimeout(() => {
            const touchY = e.touches[0].clientY - 79//手指距离顶部的高度
            const index = Math.floor((touchY - this.startY) / 20)//当前手指的字母
            if (index >= 0 && index < this.letters.length) {
              this.$emit('change', this.letters[index])
            }
          }, 16)
        }
      },
      handleTouchEnd: function () {
        this.touchStatus = false
      }
    }
  }
</script>

<style scoped lang="stylus">
  .list
    display flex
    flex-direction column
    justify-content center
    position absolute
    top 1.58rem
    right 0
    bottom 0
    width .4rem

    .item
      text-align center
      line-height .4rem
      color $bgColor
</style>
